1. Field of the Invention
The present invention relates to a magnetic recording medium in which a recording layer is formed in a predetermined concavo-convex pattern and recording elements are each formed as a convex portion of the concavo-convex pattern. The present invention also relates to a magnetic recording and reproducing apparatus provided with such a magnetic recording medium and a method for manufacturing the magnetic recording medium.
2. Description of the Related Art
Conventional magnetic recording media, such as hard disks, are sectioned into data areas and servo areas for use, and servo information for, for example, magnetic head alignment is recorded in the servo areas.
The servo area is further sectioned into a preamble area for clock synchronization, a servo address mark (SAM) area for indicating the start of servo data, a track address signal area for indicating a track number, a sector address signal area for indicating a sector number, a burst signal area for tracking a magnetic head, and the like. Servo information appropriate for the purpose is recorded in each of the above areas.
Specifically, the servo information is recorded as a predetermined servo pattern such that binary information, being a “0” or “1,” is recorded in each magnetic domain corresponding to one bit of information. In conventional longitudinal recording media, binary information of “0” or “1” is recorded by magnetizing each magnetic domain in one or the opposite circumferential direction. Alternatively, in perpendicular recording media which have become increasingly common in recent years, binary information of “0” or “1” is recorded by magnetizing each magnetic domain in one direction perpendicular to its surface or in the opposite direction.
In the step of recording such servo information, magnetic domains are sequentially magnetized based on binary information of “0” or “1” for each magnetic recording medium. Hence, low productivity tends to be a problem. In particular, in recent years, with an increase in areal density and with a decrease in the head flying height accompanying the increase in areal density, high density and high precision recording is required even for the servo information. Therefore, there are growing demands to improve the efficiency of servo information recording.
In view of the foregoing, a technique has been proposed in which, in a servo area, a recording layer is formed only in either areas for recording information of “0” or the other areas for recording information of “1” and this recording layer is formed in the shape of a servo pattern (see, for example, Japanese Patent Application Laid-Open No. Hei 6-195907). In this manner, by uniformly applying a direct current magnetic field to such a magnetic recording medium, the recording layer is magnetized so as to conform to the servo pattern. Therefore, the servo information recording efficiency can be significantly improved.
Meanwhile, a significant improvement in the areal density of magnetic recording media has been achieved by, for example, reducing the size of magnetic particles constituting a recording layer, changing materials, and improving the precision of head processing. A further improvement in the areal density is expected in the future. However, problems such as incorrect recording of information on a track adjacent to a target recording track and crosstalk during reproduction caused by the limit of magnetic head processing and by the broadening of the recording magnetic field emanating from a magnetic head have become apparent. Therefore, the improvement of the areal density by conventional improvement techniques has reached its limit.
Hence, discrete track media and patterned media have been proposed as candidates for magnetic recording media in which a further improvement in the areal density can be achieved. In the discrete track media and patterned media, a recording layer is formed in a concavo-convex pattern in a data area, and recording elements are each formed as a convex portion of the concavo-convex pattern. When such discrete track media and patterned media are manufactured, the recording elements are formed as the convex portions in the data area. Therefore, recording elements can be formed in a servo pattern in the servo area, and this is particularly advantageous in terms of productivity.
In order to obtain excellent recording-reproducing characteristics, it is considered that, preferably, the flying height of a magnetic head is kept constant so that the magnetic gap between the magnetic head and the upper surface of each recording element is held constant. Hence, it has been proposed to flatten the surface of a magnetic recording medium by filling concave portions between recording elements with a filling material such as a non-magnetic material.
As described above, the areal density of magnetic recording media has increased significantly, so that the size of magnetic domains has decreased. Therefore, in order to obtain excellent recording-reproducing characteristics, it is important to obtain a servo signal with a high signal-to-noise (S/N) ratio. In order to obtain a servo signal with a high S/N ratio, it is preferable that the direction of magnetization be clearly reversed across the boundary between a magnetic domain storing information of “0” and a magnetic domain storing information of “1.”
However, in practice, a magnetic transition region in which recorded information is recognized as neither “0” nor “1” is present around the boundary between magnetic domains. Such a magnetic transition region can be a source of noise. The circumferential length of the magnetic transition region is nearly constant irrespective of the circumferential length of the adjacent magnetic domains.
Meanwhile, as described above, since the areal density of magnetic recording media has increased significantly, so that the circumferential length of magnetic domains has decreased, the ratio of the circumferential length of the magnetic transition region relative to the circumferential length of the magnetic domains is increasing. Hence, the S/N ratio of a servo signal tends to decrease.
Moreover, in a disk-shaped magnetic recording medium such as a hard disk, its circumferential speed relative to a magnetic head is higher on the outer side in the radial direction and is lower on the inner side in the radial direction. However, the frequency corresponding to the number of information recording-reproduction processes per unit time is constant irrespective of the position on the magnetic recording medium. Therefore, the circumferential length of magnetic domains each corresponding to one bit of information is larger on the outer side of the magnetic recording medium and is smaller on the inner side. Hence, there is a problem that the S/N ratio of a servo signal is likely to decrease particularly in an inner area of a magnetic recording medium.